rackunit: make check-exn/check-not-exn raise error if given a non-thunk

Previously, (check-exn exn? 'foo) would pass even though 'foo is not a
thunk.  Now it raises an exn:fail:contract? exception.

Previously, (check-not-exn 'foo) would produce a check failure.  Now it
raises an exn:fail:contract? exception.